Rotational dynamics of confined C₆₀ from near-infrared Raman studies under high pressure

Abstract

Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. We have recently studied the rotational dynamics of C₆₀ molecules confined inside single walled carbon nanotube (SWNT) by analyzing the intermediate frequency mode lattice vibrations using near-infrared Raman spectroscopy. The rotation of C₆₀ was tuned to a known state by applying high pressure, at which condition C₆₀ first forms dimers at low pressure and then forms a single-chain, nonrotating, polymer structure at high pressure. In the latter state the molecules form chains with a 2-fold symmetry. We propose that the C₆₀ molecules in SWNT exhibit an unusual type of ratcheted rotation due to the interaction between C₆₀ and SWNT in the “hexagon orientation,” and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature.

Because of its high symmetry and relatively weak intermolecular interactions, the fullerene C₆₀ presents a nearly ideal system for studying different crystal structures possible in one-dimensionally constrained nanostructure systems, such as C₆₀ peapods [C₆₀ molecules inserted into single walled carbon nanotubes (SWNTs)] (1). Such nanostructures are emerging as a class of nanoscale materials with tunable mechanical, electronic, thermal, and optical properties (2, 3). In contrast to bulk C₆₀ crystals, C₆₀ inserted in SWNTs form monomeric linear chains in which each C₆₀ has only two nearest neighbors, as compared to 12 neighbors in crystalline bulk C₆₀ (4). As a result the intermolecular interactions in these self-assembled carbon nanostructures and the dynamic behavior of the encapsulated molecules are expected to be different from those in solid fullerenes. It is well known that the solid C₆₀ crystallizes into a face centered cubic structure at room temperature, and the molecules perform nearly free rotations (5). For the peapods, recent theoretical simulations suggest that the C₆₀s have two different orientations with low potential energy when they are trapped into SWNTs, the pentagonal and hexagonal orientations (6–8) with a pentagon and a hexagon perpendicular to the tube axis, respectively. Very recent experimental studies on peapods by inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) suggest that the linear arrays of C₆₀ molecules still rotate inside SWNTs at room temperature, either showing quasi-free rotation or being dynamically disordered with high orientational mobility (9, 10). However, how the C₆₀ molecules rotate inside SWNTs, especially at room temperature, is still an interesting open question. It is not only necessary to search another effective experimental method to study the possible rotation of confined C₆₀ in detail, but also very important to study the rotation from a different perspective. Considering that C₆₀ rotates down to very low temperature, it would be helpful to employ other techniques to change the rotational state in a way similar to the temperature effect, which can effectively change the rotational state and stop the C₆₀ molecules in a known state from which the initial rotational state can be deduced. Motivated by this idea, we have used near-infrared (NIR) Raman spectroscopy and high pressure technique to study the rotational dynamics of peapods.

Raman spectroscopy is a valuable technique for investigating the vibrational properties of C₆₀ inside SWNT (11, 12). In particular, intermediate frequency modes (IFM) are very closely related with the orientational state of C₆₀ (13), providing an approach to study the dynamics of rotation of C₆₀. To the best of our knowledge, these modes have previously only been detected for pristine peapods at very low temperature and by using high energy excitation lasers with obvious irradiation effects (14). Because NIR Raman spectroscopy is also able to detect such modes it provides us with an approach to study the rotational state of C₆₀, detect the presence of covalent polymer bonds, and determine the type of polymeric C₆₀ phase formed inside the SWNT.

In this report, we have carried out NIR excitation Raman spectroscopy to reveal the intrinsic IFM vibrations of peapods, both at room temperature and down to 100 K. The rotational state of C₆₀ was tuned to a known state by an increase of the intermolecular interaction between C₆₀ molecules induced by applying external pressure, under which condition C₆₀ first forms covalently bonded dimers, and thus slows down its free rotation, at low pressure and then forms a single-chain polymer structure with a known molecular orientation at very high pressure. By carefully comparing the changes in the IFM modes for these peapods in the various states, we propose that the C₆₀ molecules inside SWNTs exhibit a ratcheted rotation with a preferred “hexagonal orientation” due to the interaction between C₆₀ and SWNT walls.

Results and Discussion

The typical Raman spectrum of pristine C₆₀-peapods obtained using NIR excitation (830 nm) at room temperature is compared with the spectra of a bulk C₆₀ sample and of pure SWNTs in Fig. 1. The spectrum of C₆₀@SWNT can approximately be considered as a weighted sum of the spectra of C₆₀ and pure SWNT, with weak lines from the nested fullerenes and strong lines from the host SWNTs. It consists mainly of four characteristic regions: (i) the low frequency modes <200 cm⁻¹, mainly radial breathing modes (RBM) for SWNTs (15); (ii) the high-frequency range between 1,500 and 1,600 cm⁻¹, containing tangential modes (the G band) for SWNTs (15, 16); (iii) fullerene modes near 1,470 cm⁻¹, i.e., the most prominent Ag (2) mode for nested C₆₀, which is upshifted from its position at 1,469 cm⁻¹ in pristine C₆₀ to 1,474 cm⁻¹ due to the interaction between C₆₀ and tubes (15); (iv) and the intermediate frequency modes (IFMs) from 200 to 1,000 cm⁻¹. Here, we are particularly interested in the IFMs. It should be noted that these Raman peaks are not simply a superposition of the individual modes for fullerene and SWNTs, but instead there are significant splits and shifts. We first assign these modes to either C₆₀ or SWNTs or both by carefully comparing with original Raman spectra of C₆₀ and empty SWNTs from the literature (17). It is clear that the Raman modes at 582, 806, and 860 cm⁻¹ belong to the host SWNT, while the 710 cm⁻¹ peak is a combination between a broad mode coming from the SWNTs and a sharp H_g(3) mode of the C₆₀. Except for these peaks, all other modes originate from nested C₆₀. We note that the most obvious changes relative to the spectra of the pure components are the split modes of H_g(1) and H_g(2), which have been changed from 270 cm⁻¹ to 292, 306, and 326 cm⁻¹ for the H_g(1) mode and from 433 cm⁻¹ into 434 and 452 cm⁻¹ for the H_g(2) mode, while the H_g(3) (710 cm⁻¹) and H_g(4) (771 cm⁻¹) modes keep the same frequencies as in bulk C₆₀. These features are remarkably different from the results obtained using a blue excitation laser (488 nm), for which these modes were not found even at low temperatures down to 5 K (14, 18). The Raman spectra are; however, in good agreement with a recent theoretical prediction for peapods (12), indicating that we have indeed observed intrinsic IFM vibrations for C₆₀ inside SWNT by using NIR excitation at room temperature.

Fig. 1.

Raman spectra of SWNT, bulk C₆₀ and peapod samples excited by an 830-nm laser at room temperature.

The clear line splits indicate that the symmetry of C₆₀ has changed and that the molecules are not in a completely free rotational state inside SWNTs. This is consistent with recent INS and NMR measurements. The splitting is caused by the interaction between C₆₀ and its carbon environment, either the SWNT wall, the two C₆₀ neighbors, or both. We know that the upward shift of the Ag (2) mode from its intrinsic position can be related to the interaction between C₆₀s and tubes, and corresponds to an apparent compression by 1–2 GPa applied pressure, or a cooling by several tens of Kelvin from the NMR results, which implies that the interaction between C₆₀ and tubes indeed exists (9, 15). Regarding the C₆₀–C₆₀ interaction between adjacent molecules, its strength depends on the filling factor of C₆₀ inside SWNT and it is thus expected that the interaction increases with increasing filling ratio. However, theoretical simulations show that the Raman spectrum of C₆₀ is only weakly dependent on the filling ratio, while the RBM modes of the SWNTs change remarkably. This study indicates that relative to the intermolecular interaction the interaction between C₆₀ and tubes is stronger and thus plays the main role in influencing the vibrational properties of trapped C₆₀ (12). Similarly, a recent NMR study also suggests that the C₆₀ intermolecular interaction is negligibly small compared with the interaction between the C₆₀ and the SWNT (10), supporting our discussion.

Let us analyze the IFM in more detail. According to previous Raman spectroscopy results for bulk C₆₀, IFM modes <1,000 cm⁻¹ are mainly from radial modes of the C₆₀ cage and can be broadly divided into two groups, pentagonal radial modes <500 cm⁻¹ [H_g(1) and H_g(2)] and hexagonal radial vibrations above this wavenumber [H_g(3) and H_g(4)] (13). It is worth noting that in our experiment, splitting mainly occurs in pentagonal radial modes while there is no obvious change in hexagonal radial vibrations, implying that anisotropic interactions act on the rotating C₆₀. Thus, the interaction between tube and pentagons on C₆₀ is slightly larger than that between tube and hexagons at room temperature. To clearly observe this phenomenon, we further carried out Raman measurements at low temperature with the results shown in Fig. 2. It is seen that the Raman spectra are quite similar at all temperatures. A striking feature is that the relative intensity ratio between the H_g(2)/H_g(4) modes increases dramatically with decreasing temperature, clearly indicating a relative increase of the tube-pentagon interaction compared with the tube-hexagon interaction.

Fig. 2.

Temperature dependence of the vibrational properties of peapods. (Left): Raman spectra of peapods between 200 and 800 cm⁻¹ at the temperatures 100 K (solid red curve), 180 K (dashed blue curve), and 260 K (dotted black curve), respectively. Right: temperature dependence of the relative intensity ratio H_g(2)/H_g(4) for the peapods. The straight line has been fitted to the data.

As C₆₀ molecules rotate inside SWNTs, such anisotropic interactions further indicate that C₆₀s are not rotating freely but have a preferred relative orientation, in which C₆₀s are subjected to the strongest interaction with the tubes (i.e., the lowest potential energy) and thus slow down and settle into this particular state. Considering that the theory predicts that fixed fullerenes inside SWNT have either pentagon-to-pentagon orientations or hexagon-to-hexagon orientations, we believe that either the pentagon or the hexagon orientational state is the energetically favored state where C₆₀s slow down. In the hexagon orientation, the equator consists of a “zig-zag” ring of C–CC–CC–C bonds, and parts of six pentagons are close to the wall. We illustrate both orientation states in Fig. 3.

Fig. 3.

Schematic images of different molecular orientations. (A) pentagon orientation, (B) hexagon orientation, (C) polymer orientation. i, ii, iii: The calculated total energy of rotating C₆₀ in SWNTs with different diameters, (9, 9), (10, 10), and (11, 11). iv: A rotating path that covers all of the three orientations mentioned in the text; rotation from c to a to b corresponding to the horizontal axis in the energy spectra (i, ii, iii).

From our Raman results that pentagons interact much more strongly with the tube than hexagons, we conclude that C₆₀ prefers to ratchet in the hexagonal orientation, where pentagons have a stronger interaction with the tube than hexagons, and that the hexagon-to-hexagon orientation is the most likely favorable state.

We calculate the energy difference with all possible orientations of C₆₀ in SWNTs with different diameters. SWNT (9, 9), (10, 10), and (11, 11) have been chosen because the corresponding diameters are similar to the peapod in our sample (19). It turns out that for C₆₀ in SWNTs the energy varies very little with the orientation. However, the hexagonal orientation has the lowest energy, as shown in Fig. 3, indicating that C₆₀s rotate in the SWNTs and prefer to ratchet in the hexagonal state, which is consistent with our experiment.

To further confirm the hexagon-to-hexagon orientation, we forced the rotational state to change by introducing covalent bonds between two adjacent C₆₀s in SWNTs. The A_g(2) mode of nested C₆₀ is down-shifted from 1,474 cm⁻¹ to 1,469 cm⁻¹ for dimers, and further to 1,465 cm⁻¹ for the single-chain polymer as discussed in detail in our previous work (15). The dimerization process was induced by applying a gentle pressure of 5 GPa in our study. The IFM modes of the Raman spectrum for dimeric peapods are shown in Fig. 4B, and for comparison we show the corresponding data for pristine peapods in Fig. 4A and for the chain polymer peapods in Fig. 4C. It is important to notice that the hexagon radial modes H_g(3) and H_g(4) show specific fingerprint changes after dimerization, such that the original lines split into lines at 710 + 725 cm⁻¹ and 754 + 771 + 779 cm⁻¹, respectively, due to formation of covalent bonds between C₆₀ molecules (arrows). This feature has not been observed in monomeric peapods but is in good agreement with the split in bulk C₆₀ dimers except for the line at 725 cm⁻¹, which might be derived from the H_g(3) mode, and changed due to interaction between C₆₀ and SWNT. At the same time the pentagon radial modes H_g(1) and H_g(2) are almost the same as in monomeric peapods except that their intensities are significantly enhanced and that the H_g(1) mode splits even further with a peak at 267 cm⁻¹, which is also a characteristic feature in dimeric bulk C₆₀. This indicates that the interaction between C₆₀ and SWNT was also enhanced during the dimerization process, in agreement with the fact that the molecular orientation implies that both hexagons and pentagons now directly face the wall (Fig. 3C). We also observe a significant enhancement of the formerly weak modes at 362 and 389 cm⁻¹, which are not visible in bulk C₆₀ dimer, nor for pristine SWNT. These two modes are related to the filling factor of C₆₀ in SWNT from theoretical calculations, such that the higher the filling factor, the stronger are these modes (12). Another peak appears at 642 cm⁻¹, and might be the infrared active vibration mode H_u(3) of C₆₀. These features show that C₆₀ dimers inside peapods feel a different interaction with their environment than those in bulk C₆₀. This tendency can be understood from the fact that fullerenes with pentagon-to-pentagon orientations cannot dimerize (polymerize) and that only hexagon-to-hexagon orientation can bring double bonds into sufficient close proximity to give rather easy dimerization (20).

Fig. 4.

Raman spectra and schematic images. Pristine peapod sample (A), of peapods containing C₆₀ dimers (B), and of peapods containing single-chain polymer (C). Squares, characteristic peaks of pristine peapods; star, a peak only detected for peapods containing dimers or polymers; arrows point to vibration peaks, which are also observed in bulk C₆₀ polymer crystal.

Based on the above studies, we thus tentatively propose three possible types of rotational dynamics for the confined C₆₀: (i) quasi-free rotation with a preferred hexagon-to-hexagon orientation state, i.e., ratcheted rotation; (ii) uniaxial rotation with hexagonal symmetry; that is, a hexagon oriented perpendicular to the tube axis; (iii) vibrational rotation, similar to the situation in the simple cubic (SC) phase of bulk C₆₀, in which one of the preferred orientations is the hexagonal one. Because recent INS results rule out the third case (10), we consider below the first and second possibilities.

To distinguish between the two possible types of rotational dynamics, we then linked further C₆₀s together, forming single chain polymeric structures inside the peapods by applying a higher pressure, 23 GPa. The Raman spectrum obtained after this treatment is shown in Fig. 4C. It is clearly seen that the spectrum has many similarities to that for the dimer peapod. There is no significant change in the IFMs of C₆₀, except for a sharp peak at 443 cm⁻¹, which might be attributed to the shifted H_g(2) mode found to be characteristic for the single-chain polymeric phase. This is normally observed at 454 cm⁻¹ but could be shifted due to encapsulation inside SWNT (21). A noticeable change was also found in the relative intensity at 400–500 cm⁻¹ near the H_g(2) mode, which might be due to pressure induced shortening of nanotubes (22) or to defects created in the SWNT wall during the pressure-induced polymerization process (23). A similar effect has been found in C₇₀ peapods (24). We know that monomeric C₆₀s may possibly rotate uniaxially around the tube axis, but such uniaxial rotation should be difficult, if not impossible, for a long chain of polymeric C₆₀. For dimers uniaxial rotation might still be possible, but because of the similarities between the spectra for peapods containing dimers and longer chains we conclude that the second case, the uniaxial rotation model, probably is not correct.

After ruling out the uniaxial rotation, we thus think that the most probable rotation mode of the C₆₀ molecules confined inside SWNT is a quasi-free rotation with preferred “hexagonal orientation”; that is, ratcheted rotation. Our result does not contradict previous experimental data, including INS and NMR studies, and can be used to explain these data very well. Another important point is that our data can also help us to interpret the polymerization process of C₆₀ in SWNTs. The polymer has been effectively generated inside the peapod under high pressure at room temperature or at elevated high temperature (20). The synthesis of such a one-dimensional chain polymer is different from that of the bulk chain polymer (orthorhombic phase), where nearest neighbor C₆₀ molecules link up along the (110) direction in an fcc lattice, a process that can be induced only at high temperature and high pressure. It is not surprising that low-temperature polymer synthesis is possible in peapods, because the C₆₀ molecules in peapods prefer to stop at the hexagon orientation, which is advantageous for the formation of covalent bonds between C₆₀ molecules (20). Recently, the authors became aware of an NMR study on the dynamic properties of C₆₀ inside SWNT (29). We recommend reading this article as a complement to our work.

Conclusions

In summary, we have experimentally studied the rotational dynamics of C₆₀ molecules in C₆₀@SWNTs peapods by using NIR Raman spectroscopy of the IFMs. The rotational state of C₆₀ was tuned to a known state by applying high pressure, at which C₆₀ molecules first form covalently bonded dimers and slow down their free rotation at low pressure, then form single-chain polymer structures with 2-fold symmetry at high pressure. The C₆₀ molecules in nanotubes exhibit an unusual type of ratcheted rotation hindered by hexagon orientation. The characteristic vibration of ratcheted rotation becomes more obvious with decreasing temperature. This study has important implications for the future utilization of peapods in nano-materials based on this type of carbon structures, and increased our basic understanding of the dynamics of a unique dimensionally constrained crystal structure. In addition, it has provided an effective method to understand confinement effects, which are important for many applications in various fields, such as superhard and electronic materials, geology, and geophysics.

Methods

The peapod sample was prepared using a vapor phase reaction synthesis method, as reported previously elsewhere. The filling factor of C₆₀ inside SWNT is >80% (15, 25). The dimer and single-chain polymer structures were induced by applying high pressures, 5 GPa and 23.5 GPa, respectively, at room temperature using a diamond anvil cell with Ar as the pressure medium as shown in our previous work (15, 26). These pristine and polymeric peapod samples were characterized by using Raman spectroscopy (Renishaw inVia) with a NIR (830-nm) excitation laser at room temperature. The measurements on pristine peapods were performed in a range of temperatures varying from 300 to 100 K. We used the Discover module of the Materials Studio software (Accelrys Software, Inc.), to calculate the energy difference with all possible orientations of C₆₀ in SWNTs with different diameters, we built our model on the experimentally suggested parameters (27). The COMPASS (28) force field is used.